MINOR: server: Improve log message sent when server address is updated

When the server address is set for the first time, the log message is a bit ugly
because there is no old ip address to report. Thus in the log, we can see :

  PX/SRV changed its IP from  to A.B.C.D by DNS additional record.

Now, when this happens, "(none)" is reported :

  PX/SRV changed its IP from (none) to A.B.C.D by DNS additional record.

This patch may be backported to 2.2.
diff --git a/src/server.c b/src/server.c
index 56ee6a1..ee743e7 100644
--- a/src/server.c
+++ b/src/server.c
@@ -3516,6 +3516,9 @@
 		case AF_INET6:
 			inet_ntop(s->addr.ss_family, &((struct sockaddr_in6 *)&s->addr)->sin6_addr, oldip, INET6_ADDRSTRLEN);
 			break;
+		default:
+			strcpy(oldip, "(none)");
+			break;
 		};
 
 		/* copy new IP address in a string */